1. Field of the Invention
The present invention relates to digital subscriber line (DSL) transmission systems which allow high speed communication on twisted pair telephone lines, in particular. The invention relates more specifically to a solution for maintaining orthogonality of a local echo.
2. Discussion of the Related Art
FIG. 1 very schematically shows a DSL transmission system at one end of a telephone line 10. A serial stream of outgoing digital data Dout is provided to a serial-to-parallel converter 12 which extracts N multibit values, each corresponding to a complex frequency domain coefficient. These frequency domain coefficients are provided to an inverse fast Fourier transform (IFFT) circuit 14 which generates, for each set of N coefficients, a time domain symbol. A symbol is thus the sum of N sinusoidal subcarriers of different frequencies, the amplitude and phase of each subcarrier being determined by the corresponding frequency domain coefficient received by the IFFT circuit.
Each symbol is transferred onto telephone line 10 through a hybrid line interface 16. Line interface 16 also receives incoming symbols from line 10. These incoming symbols are provided to a fast Fourier transform (FFT) circuit 18 which extracts the N frequency domain coefficients of each received symbol. These frequency domain coefficients are arranged into an incoming serial data stream Din through a parallel-to-serial converter 20.
A similar structure is provided at the other end of line 10.
The illustrated serial-parallel conversions are for clarification purposes only, since the FFT and IFFT circuits operate, in practice, directly on the serial streams in a pipeline manner.
FIG. 2 illustrates the spectrum of the signals conveyed on line 10 in such a system. The bandwidth of the telephone line is subdivided in N channels, each corresponding to one frequency domain coefficient as processed by the IFFT and FFT circuits. There are for example N=2048 channels, each having a bandwidth of 5 kHz. The first frequency domain coefficients may be 11 bits wide and thus correspond to 2048 different amplitudes/phases in the time domain. Because of the attenuation undergone by the higher frequencies on the telephone line, the last frequency domain coefficients may only be two bits wide.
A gap shown in FIG. 2 at the beginning of the spectrum is reserved for “plain old telephone services” (POTS).
FIG. 3 shows the spectrum of one subcarrier f1 conveyed in a symbol. This spectrum is not a discrete value at f1 because the symbol is sampled in a window. The power spectral density thus has a sin(x)/x (sinc) function shape. To avoid the interference of the lobes of this spectrum with nearby channels, the subcarriers are chosen to be “orthogonal”. This means, as shown, that the distance between subcarriers is chosen so that each subcarrier is at a zero crossing of the sinc functions. The sinc functions only depend on the window width, which is constant, and thus have a constant pseudo period and all have zero values at the same frequencies.
In order to avoid near-end and local echo problems, the channels are often used only for unidirectional communication, whereby, in theory, near-end echoes of outgoing symbols occur in channels not used for the incoming symbols. In practice, there is a main difficulty in implementing this solution, as explained below.
FIG. 4 shows a stream of outgoing symbols S1, S2 . . . on line 10, and a stream of incoming symbols S′1, S′2 . . . For clarity reasons, it is supposed that the incoming symbols convey only one subcarrier f1 and the outgoing symbols convey only one subcarrier f2, the subcarriers f1 and f2 being adjacent. As shown, the incoming and outgoing symbols are not synchronized, the phase-shift depending essentially on the characteristics of the telephone line 10. The FFT circuit 18 synchronizes its sampling on the incoming symbols, as shown by dotted lines. While each incoming symbol is sampled by FFT circuit 18, the echo of the outgoing signal is also sampled.
Due to the delay between the incoming and outgoing symbols, the transitions between the outgoing symbols will be sampled in the echoes. Such transitions are discontinuities which have a wide spectrum in the frequency domain. This is shown in FIG. 3 by dotted lines for subcarrier f2. This wide spectrum affects all the nearby channels.
FIG. 5 illustrates a conventional solution, disclosed in PCT patent application WO 97/06619, for avoiding this problem. Each symbol is extended by a cyclic suffix CS which is longer than the delay between the incoming and outgoing symbols. This cyclic suffix is a simple copy of the first portion of the corresponding symbol. (The cyclic suffix mentioned here should not be mistaken with a cyclic prefix conventionally used for other purposes. Such a cyclic prefix, not shown for clarity purposes, corresponds to an end-portion of each symbol placed in front of the symbol.)
As shown, the incoming symbols are sampled outside the cyclic suffixes CS. In this manner, the system samples the echo of a portion of each outgoing symbol followed by a portion of its cyclic suffix CS, and does not see a transition between two symbols. As a consequence, the sampled echo remains orthogonal, i.e. each subcarrier in the sampled echo has the sinc-shaped spectrum shown in continuous line in FIG. 3 and not the wide spectrum shown in dotted line.
A drawback of this solution is that it needs the transmission of a cyclic suffix for each symbol, which suffix is a duplication of a portion of the corresponding symbol, often 5% of the symbol. This inevitably reduces the transmission throughput of the system by at least 5%.
It could be devised to cancel the local echo in order to omit the use of a cyclic suffix. Moreover, a local echo canceling would allow each channel to be used in full duplex mode, i.e. in both transmission directions.
Theory on how to design adaptive filters, such as Finite Impulse Response filters (FIR) used in echo-cancellers, is provided in the manual “Adaptive Filter Theory” by S. Haykin, Prentice-Hall, New Jersey, 1991 which is incorporated herein by reference.
The use of an echo-canceller would dramatically increase the throughput of the system but the hardware requirements may increase more than the throughput gain obtained. Indeed, an adaptive filter used for echo-cancellation in a DSL transmission system should be designed to store all the time domain samples of a currently processed symbol and for adaptively calculating weighting coefficients for a large number of samples, reaching 1000 if the symbol has more than 1000 samples. The complexity of the algorithm is proportional to the product of the size of the filter by the number of weighting coefficients to calculate, since all the weighting coefficients are recalculated each time a new sample is received. The number of samples of a symbol is equal to double the order of the FFT, i.e. the number of channels used.